Historically, halide-containing raw materials, such as, SiCl.sub.4 or mixtures of SiCl.sub.4 with various dopants, have been used in the manufacture of preforms by vapor phase deposition techniques, such as, the VAD (vapor axial deposition) and OVD (outside vapor deposition) techniques.
In these procedures, the halide-containing raw materials are volatilized and then hydrolyzed in a burner to produce soot particles which are collected on a rotating starting rod (bait tube) in the case of VAD or a rotating mandrel in the case of OVD. In some OVD systems, the cladding portion of the preform is deposited on a previously-formed core preform, rather than on a mandrel.
Numerous burner designs have been developed for use in such processes, examples of which can be found in Moltzan et al., U.S. Pat. Nos. 3,642,521, Powers, 4,165,223, Moltzan, 3,565,345, and Moltzan, 3,698,936.
In particular, Suda et al., U.S. Pat. No. 4,801,322, discuss the problem of soot particles adhering to the orifices of a burner (see column 12, lines 55-66) Kawachi et al., U.S. Pat. Nos. 4,406,684, and Watanabe et al., 4,915,717, disclose burners in which an outlet for a raw material is surrounded by an outlet for an inert gas; and Andrejco et al., U.S. Pat. No. 4,474,593, disclose a burner employing concentric tubes, some of which have machined splines on their outer surfaces.
The use of halide-containing raw materials generates substantial quantities of halide-containing by-products, e.g., hydrochloric acid. To avoid environmental pollution, these by-products must be collected, which increases the overall cost of the preform production process. Accordingly, halide-free materials and, in particular, halide-free, silicon-containing materials are desirable starting materials for the production of preforms. See Dobbins et al., U.S. Pat. No. 5,043,002.
As explained in the Dobbins et al. patent, the relevant portions of which are incorporated herein by reference, particularly preferred halide-free, silicon-containing materials for use in producing preforms are polymethylsiloxanes, with the polymethylcyciosiloxanes being particularly preferred, and with octamethylcyclotetrasiloxane (OMCTS) being especially preferred. These same halide-free, silicon-containing raw materials are preferred for use with the present invention.
Although conventional burners can be used to oxidize halide-free, silicon-containing materials, such burners, having been designed for use with halide-containing raw materials, do not achieve particularly efficient utilization of halide-free materials. These deficits arise both from the burner's construction and from the exit locations at the burner's face of the various gases used in the oxidization process. As discussed in detail below, in accordance with the present invention, gas flow patterns and burner configurations have been developed which eliminate these deficits.
In the OVD process, preforms can be made by traversing a single burner along the entire length of the preform or by using a series of burners mounted on an oscillating manifold, with each burner traversing only a portion of the preform. In the latter case, the uniformity of the burners relative to one another determines the longitudinal uniformity of the preform.
Prior art burners have often exhibited substantial variability in their properties which, in turn, has resulted in an undesirable level of variability in preforms produced using such burners in an oscillating manifold. As detailed below, as a further aspect of the invention, burner configurations have been developed which minimize this burner variation.